(1) Field of the Invention
The present invention relates to a system for exhaust gas purification, capable of effectively purifying the harmful substances present in the exhaust gas emitted from an internal combustion engine, particularly the hydrocarbons discharged from the engine in a large amount during the cold start.
(2) Description of Related Art
Active researches and developments have heretofore been made on systems for exhaust gas purification, used for purifying the harmful substances present in exhaust gas from automobile engine, such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and the like. In recent years, as the regulation on exhaust gas has become stricter, the purification of HC during engine start (cold start) has become an important technical task.
The reason is as follows. During the cold start of engine when the temperature of exhaust gas from engine is low, the catalyst disposed in exhaust pipe of engine does not reach its light-off temperature and has a low purification ability. Moreover, during this period, as compared with the period of continuous operation of engine, a large amount of HC is discharged from the engine. As a result, the HC discharged during the cold start occupies a large proportion of the total harmful substances discharged from the engine and needs to be removed.
As one means for achieving the above-mentioned technical task (the purification of HC during engine start), there was proposed a system for exhaust gas purification disposed in the exhaust pipe of an internal combustion engine, comprising (1) an absorbent containing a component (e.g. zeolite) having an adsorptivity for HC and (2) a catalyst provided downstream of the adsorbent (1). This adsorbent is intended to adsorb the unburnt HC discharged from the engine in a large amount during the cold start, temporarily from the start of catalyst heating to the start of catalyst light-off.
The HC adsorbed on an adsorbent begins to be desorbed from the adsorbent as the temperature of the adsorbent is elevated owing to the heat of exhaust gas, etc. The temperature at which the HC begins to be desorbed from the adsorbent, is lower than the light-off start temperature of catalyst; therefore, the desorption of HC begins before the light-off start of catalyst. Consequently, the most part of the HC desorbed from the adsorbent during the period from the HC desorption start of adsorbent to the light-off start of catalyst is discharged into the air without being purified. Hence, in the exhaust gas purification system comprising an adsorbent, it is necessary that the time span from the timing at which the adsorbent reaches the HC desorption start temperature, to the timing at which the catalyst reaches the light-off start temperature, is minimized in order to minimize the amount of the HC discharged without being purified.
As one means for minimizing the above-mentioned time span, there can be mentioned a technique of increasing the HC desorption start temperature of adsorbent. As one such technique, there is disclosed, in Japanese Patent Application Kokai (Laid-Open) No. 10613/1996, an adsorbent of higher HC desorption start temperature, obtained by allowing a zeolite to contain at least one kind of ion of an element having an electronegativity of 1.40 or more. Also in Japanese Patent Application Kokai (Laid-Open) No. 10566/1996, it is disclosed that a catalyst-adsorbent containing at least one noble metal selected from Pt, Pd and Rh and a zeolite can give a higher HC desorption start temperature by allowing the zeolite to contain at least one kind of ion of an element having an electronegativity of 1.40 or more.
As other means for minimizing the time span, there can be mentioned a technique of slowing the temperature elevation of adsorbent. For example, Japanese Patent Application Kokai (Laid-Open) No. 5-59942 discloses a cold HC remover comprising an HC adsorbent and a three-way catalyst provided upstream of the adsorbent. In this apparatus, the temperature elevation of the HC adsorbent is slowed because the three-way catalyst provided upstream of the HC adsorbent absorbs the heat of exhaust gas and thereby the gas temperature at the adsorbent inlet is kept low for a relatively long time.
The adsorbent disclosed in Japanese Patent Application Kokai (Laid-Open) No. 8-10613 and the catalyst-adsorbent disclosed in Japanese Patent Application Kokai (Laid-Open) No.8-10566, certainly have a higher HC desorption start temperature as compared with ordinary adsorbents using a zeolite. However, they have had a problem in that they have insufficient heat resistance and easily undergo thermal deterioration. Further, the technique disclosed in Japanese Patent Application Kokai (Laid-Open) No. 5-59942 are unable, at times, to sufficiently exhibit the intended effect of slowing the temperature elevation of HC adsorbent, because no consideration is made to the volume or heat capacity of the three-way catalyst provided upstream of the HC adsorbent.
The present invention has been completed in view of the above situation and is intended to provide a system for exhaust gas purification where the thermal deterioration of the adsorbent is reduced because there is used an adsorbent of higher HC desorption start temperature and because the thermal load applied to the adsorbent is decreased by the use of a particular means.
According to the present invention, there is provided a system for exhaust gas purification (the first system for exhaust gas purification) disposed in the exhaust pipe of an internal combustion engine, which comprises:
an adsorbent formed by loading, on a monolithic carrier, (1) a zeolite containing at least one kind of ion of an element having an electronegativity of 1.40 or more and (2) a catalyst material formed by loading at least one kind of noble metal selected from Pt, Pd and Rh on a heat-resistant inorganic oxide, and
at least one loaded carrier formed by loading, on a monolithic carrier, a catalyst component having a purifiability for the harmful substances present in the exhaust gas emitted from the engine and/or an adsorbent component having an adsorptivity for the hydrocarbons also present in the exhaust gas, the loaded carrier being provided upstream of the adsorbent in the flow direction of the exhaust gas and having a total volume of 0.6 l or more.
According to the present invention, there is further provided a system for exhaust gas purification (the second system for exhaust gas purification) disposed in the exhaust pipe of an internal combustion engine, which comprises:
an adsorbent formed by loading, on a monolithic carrier, a zeolite containing (1) at least one kind of ion of an element having an electronegativity of 1.40 or more and (2) at least one kind of noble metal selected from Pt, Pd and Rh, and
at least one loaded carrier formed by loading, on a monolithic carrier, a catalyst component having a purifiability for the harmful substances present in the exhaust gas emitted from the engine and/or an adsorbent component having an adsorptivity for the hydrocarbons also present in the exhaust gas, the loaded carrier being provided upstream of the adsorbent in the flow direction of the exhaust gas and having a total volume of 0.6 l or more.
According to the present invention, there is furthermore provided a system for exhaust gas purification (the third system for exhaust gas purification) disposed in the exhaust pipe of an internal combustion engine, which comprises:
an adsorbent formed by loading, on a monolithic carrier, a zeolite containing at least one kind of ion of an element having an electronegativity of 1.40 or more,
a catalyst formed by loading, on a monolithic carrier, a catalyst material formed by loading at least one kind of noble metal selected from Pt, Pd and Rh on a heat-resistant inorganic oxide, the catalyst being provided downstream of the adsorbent in the flow direction of the exhaust gas emitted from the engine, and
at least one loaded carrier formed by loading, on a monolithic carrier, a catalyst component having a purifiability for the harmful substances present in the exhaust gas emitted from the engine and/or an adsorbent component having an adsorptivity for the hydrocarbons also present in the exhaust gas, the loaded carrier being provided upstream of the adsorbent in the flow direction of the exhaust gas and having a total volume of 0.6 l or more.
FIG. 1 is a schematic drawing showing the system for exhaust gas purification used in Example 1 or 14.
FIG. 2 is a schematic drawing showing the system for exhaust gas purification used in Example 2.
FIG. 3 is a schematic drawing showing the system for exhaust gas purification used in Example 3.
FIG. 4 is a schematic drawing showing the system for exhaust gas purification used in Example 4.
FIG. 5 is a schematic drawing showing the system for exhaust gas purification used in Example 5.
FIG. 6 is a schematic drawing showing the system for exhaust gas purification used in Example 6.
FIG. 7 is a schematic drawing showing the system for exhaust gas purification used in Example 7.
FIG. 8 is a schematic drawing showing the system for exhaust gas purification used in Example 8.
FIG. 9 is a schematic drawing showing the system for exhaust gas purification used in Example 9.
FIG. 10 is a schematic drawing showing the system for exhaust gas purification used in Example 10.
FIG. 11 is a schematic drawing showing the system for exhaust gas purification used in Example 11.
FIG. 12 is a schematic drawing showing the system for exhaust gas purification used in Example 12.
FIG. 13 is a schematic drawing showing the system for exhaust gas purification used in Example 15.
FIG. 14 is a schematic drawing showing the system for exhaust gas purification used in Comparative Example 1.
FIG. 15 is a schematic drawing showing the system for exhaust gas purification used in Comparative Example 2.
FIG. 16 is a schematic drawing showing the system for exhaust gas purification used in Comparative Example 3.
FIG. 17 is a drawing showing an electrically-heated catalyst.